Stock flow control system for feeding a paper machine headbox

ABSTRACT

Stock feed to a paper machine headbox is controlled by a stock (fan) pump which is controlled to run at variable speeds, the input of stock to the pump originating from a thick stock containing conduit and a conduit containing mixing water. The speed of the pump motor is controlled by a signal from a control element which is responsive to measurements of the stock level or pressure within the headbox and an adjusted value of the forming wire speed. In addition, a thick stock pump located in the thick stock conduit is adjusted to run at a variable speed by a control device which is responsive to signals indicating the pressure drop across an orifice in the thick stock conduit and a signal based on the web speed. Alternatively, the thick stock pump may run at a constant speed and the control device can be used to control an orifice opening or throttling device located in the thick stock conduit between the pressure-drop determining orifice and the stock (fan) pump.

United States Patent 1191 3,779,863 Wahren Dec. 18, 1973 1 STOCK FLOW:CONTROL SYSTEM FOR FEEDING A PAPER MACHINE HEADBOX lnventor:

Assignee:

Filed:

App]. No.:

Douglas Wahreln, Taby, Sweden Allmanna Svenska Ele cktriskaAktiebolaget, Vasteras, Sweden Feb. 12, 1970 Foreign ApplicationPriority Data Feb. 18, 1969 Sweden 2196/69 US. Cl 162/253, 162/258,162/259,

Int. Cl. n21: l /06,D21f l/08 Field of Search 162/336, 339, 259,

References Cited UNITED STATES PATENTS Taylor Primary Examiner-S. LeonBashore Assistant Examiner-Richard H. Tushin Attorneyl(enyon and KenyonReilly Carr and Chapin [5 7] ABSTRACT Stock feed to a paper machineheadbox is controlled by a stock (fan) pump which is controlled to runat variable speeds, the input of stock to the pump originating from athick stock containing conduit and a conduit containing mixing water.The speed of the pump motor is controlled by a signal from a controlelement which is responsive to measurements of the stock level orpressure within the headbox and an adjusted value of the forming wirespeed. In addition, a thick stock pump located in the thick stockconduit is adjusted to run at a variable speed by a control device whichis responsive to signals indicating the pressure drop across an orificein the thick stock conduit and a signal based on the web speed.Alternatively, the thick stock pump may run at a constant speed and thecontrol device can be used to control an orifice opening or throttlingdevice located in thethick stock conduit between the pressure-dropdetermining orifice and the stock (fan) pump.

4 Claims, 2 Drawing Figures INYENTOR. DOUGLAS WAHREN STOC K FLOW CONTROLSYSTEM FOR FEEDING A PAPER MACHINE HEADBOX BACKGROUND OF THE INVENTIONThe invention presented here refers to an arrangement for stockproportioning of paper machines, comprising a thick stock pump forfeeding pulp to the system, which feeds a headbox system, from whichstock is delivered on to a wire or wet felt. Such a system consists of awhite-water chest, fan (mixture) pump, piping and, possibly, cleaningdevices as well as a headbox, from which the stock suspension dilutedwith whitewater is uniformly distributed over the width of the machineon a wire or wet felt.

A usual arrangement for such stock proportioning is shown in FIG. 1, andcomprises a level box 1 provided with an overflow (by the arrow 2) orsome other form of lever control, arranged at a certain height over aproportioning point 3, where the driving pressure for the thick stockflow is determined by the difference between levels in the levelbox-andwhite-water chest. Water, for example, previously used water(white-water, arrow 4) is fed to the proportioning point 3, and thickstock is pumped to a levelbox (at 5). With the object of obtaining aconstant stock flow (constant basisweight) to the headbox and from thereto the paper machine, the incoming thick stock flow is controlled bymeans of a manual or automatic, for example, computer-controlled, valve6. The accuracy requirement means that the levelbox must normally be setup at a height of 4-8m over the white-water level so as to obtain thenecessary pressure drop across the basis-weight valve. This system willconsequently take up considerable space and will be expensive in view ofthe piping involved etc. This may lead to disturbances, varying degreeof blocking up of the valve owing to the low pressure drop available.Setting up of the levelbox at a considerable height over theproportioning position will require a higher building (insulation, etc.)which can seldom be accomplished. In such a system the influence offriction in pipes may be aggravated and, furthermore, changes in theproperties of the stock, for exam ple, beating degree, temperature ordegree of cooking, cannot be simply measured or calculated. This meansthat difficulties may arise with automatic control of the basis-weightvalve. It is also difiicult to achieve a constant stockflow owing to thepresence of spontaneous flow variations caused by, for example, airadmixture, wave phenomena due to the free liquid surface in thelevelbox, and fluctuations in the content of additives. The influence ofthe air is important. A level variation of 3 cm at a head of, forexample, 5 in will lead to a change in the basis-weight of only 0.2percent. Variation of the air content leads to an altered resistance inthe piping, pressure loss due to friction and thus a considerablyaltered proportioning. Discontinuity in the piping and accumulation ofair are normal phenomena occurring in certain designs of the levelbox.In conjunction with stock proportioning it has been considered desirableto relate this to the wire speed so that a constant basisweight of thepaper is maintained for changes in the machine speed or slice outflowfrom the headbox. Hitherto it has been difficult to satisfy theserequirements, owing to the difficulty of moving the levelbox, whichnecessitates controlling of the basisweight valve, whose characteristicsmust then be stored in a computer, and to the insufficient developmentsin measuring techniques in this respect.

In the type of application treated here the measuring accuracyrequirements are very severe, since measuring errors lead tobasis-weight variations, which in their turn result in magnifiedmoisture content variations in the finished paper. A relatively smallvariation in the basis-weight results in a relatively large variation inthe moisture content. As an example of the order of magnitude involvedit can be mentioned that a transient change in the stock' flow of 1percent results in a transient change in the moisture content of 2percent in absolute dimensions, i.e., 25 percent in relative dimensions.

The demand imposed on flowmeters can be expressed in such a way thattheir measuring accuracy must be considerably better than thespontaneous flow fluctuations, which the flow control is intended toreduce on the basis of signals from these instruments. An acceptablevalue of the variations between the maximum and the minimum values ofthe moisture content may be i 0.5 percent, which means thatcorresponding variations for the stock flow must be less than :t 0.25percent, i.e., a value that is difficult to achieve with conventionalinstruments of the magnetic flow-meter type.

It has been experimentally found that this necessary accuracy can beachieved by the measurement of either the difi'erential pressure betweenmaximum and minimum pressure within a comparatively short Venturi tubeor the pressure drop across a suitably arranged oriiice, in which therelationship of the flow rate with the network strength of the stock issufficiently high, without however, cavitation phenomena occurring. Ifthe necessary flow rate is to be achieved, a larger pressure drop willbe required than that usually obtained with the levelbox arrangementshown in principle in FIG. 1. When the arrangements with very long pipesoutlined above are used, instability easily occurs owing to thehydrodynamic properties of the stock and/or air pockets.

. The invention presented here refers to an arrangement for solving theabove-mentioned and other problems and is of the type mentioned in thefirst paragraph of this application. The main feature of this is that anorifice is arranged in the stock line to the headbox. The pressure dropacross this orifice is measured and the stock flow is controlled on adesired value for this pressure drop, which is usually proportional tothe square of the web (wire) speed.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 2 shows the basic principleof this invention. A headbox 12, to which pulp and water (stock) ispumped via a pump 13 (see arrow 14) located by the inlet side, isarranged in the usual manner at the point to the headbox, in this caseto the inlet of the mixture pump 13. The pressure drop across theorifice is measured in the normal way for such devices, and the signalfor this pressure drop is fed to a control device 19 for the prime mover21 driving the thick stock pump 17. The prime mover 21 may be a dc.motor controlled by an S.C.R. converter or by some other similar means,but it may also be an a.c. commutator motor or a hydraulic drive motor,where the supply of the drive medium can be controlled, and thus alsothe motor speed. The signal (at 20) gives the actual value of thepressure drop across the orifice 18 and the speed of the motor (21) iscontrolled on a desired value, which is determined by the machine speedand possibly also the basis-weight as described below.

The speed control of motor 21 can be replaced by a motor intended to runat constant speed and supplemented by an extra orifice 22 for the thickstock flow. This orifice is adjustable and, when its setting is changed,it alters the the pressure drop across the orifice 18.

In addition to the solution of the above-mentioned problems, this systemhas the advantage in both these cases that, besides there being hardlyany air admixture in the stock (unlike the case where free liquidsurfaces occur or an overflow is used, see the FIG. 1), the lines 23 canbe arranged vertically, which prevents the accumulation of air normallyoccurring with descending pulp flows.

The speed at the end of the web, shown here schematically by roll 37, ismeasured with a tacho-generator 38, whose output signal is squared in asquaring element 39. The output signal of the latter in its turn will beproportional to the square of the web speed. A suitable pulpproportioning, for example, suitable basisweight is set with aproportionalizing element such as a potentiometer 29, and thisproportioning (basisweight) signal 30 is fed to the control element 19an will constitute the desired value for the pressure drop across theorifice 18. This value is thus adjusted for altered motor speed (at 21)or altered throttling (at 22) and therefore altered proportioning (ofstock to the paper machine). The desired-value signal 30 will thereforebe proportional to the square of the web speed (v and is thus altered asthe machine speed changes. The speed of the wire 11 (not exactly thesame as v) is measured in a suitable manner by means of a tachogenerator24 or by some other conventional means at the drive 25 of the wire. Thismeasuring element gives 26 a signal proportional to the wire speed (v,)at 11. This signal 26 is fed to a squaring element 27, whose out-putsignal is therefore proportional to the square of the speed (v,).

The v, signal can also be taken out across another proportionalizingelement 31, for example, a potentiometer, which is set to a desiredvalue of the square of the slice outflow quotient 4: i.e., the desiredratio between the slice outflow velocity and the wire speed. An actualvalue for these quantities can be obtained from the pressure or levelsignals in the headbox, which are measured at 32 and 33 and added in thecontrol element 34 with the desired value (from 31) to form an errorsignal, which is used for controlling the drive 35 of a stock pump 13for controlling the desired slice outflow pressure (arrow 36) or outflowquotient. An altered web speed thus also results here in an altereddesired value and thus changing of the pump speed.

where y is the flow velocity in m/s, 1 the network strength of the stockin Newton/m and 4) the density of the suspending medium (water), approx.l0 kg/m. At the same time Reynolds number (Re) must be Re v D dB/u5,000,

where D is the diameter of the measuring element in m and p. the dynamicviscosity (kg/m/s).

Cavitation in the measuring element should be avoided (this imposes'apractical upper limit). The pressure value across the orifice 18 canalso be controlled on a value proportional to the total-head in theheadbox, comprising fluid (stock) pressure plus pressure of any aircushion, plus possibly a negative pressure addition for vacuum runningat low speeds.

The flow measurement can also be accomplished indirectly by measurementof the head of pump 17 and the pump speed (at 21) and combining of thesevalues. Examples according to the above can be varied in different wayswithin the framework of the following patent claims.

What is claimed is:

1. Device for proportioning the pulp suspension for paper machines,comprising feed conduits for pumping pulp and water to a mixture pumpfor pumping pulp mixed with water (stock) to a headbox, from which(headbox) the stock is delivered to a wire for further transport ofstock, the driving means of the mixture pump being controlled from acontrol element responsive to signals from a measuring means within theheadbox measuring the level or pressure in the same and to signals frommeans responsive to the wire speed, one of said conduits being a pulpconduit provided with a thick stock pump for pumping pulp to saidmixture pump to be mixed with water from another of said conduits beforesaid mixture pump, in said pulp conduit being arranged an orifice forobtaining a pressure drop and means for measuring said pressure drop andcontrol means for controlling the driving means for said thick stockpump, a reference value for desired basis weight for the suspensionbeing obtained in reference means responsive to the produced web speed,the output of which means is fed to the input of the control means ofthe thick stock pump together with the output from the pressure dropmeasuring means, the difference between pressure drop and referencevalues controlling the feeding rate of said thick stock pump.

2. Device according to claim 1 characterized in that said orifice forobtaining a pressure drop comprises a Venturi tube.

3. Device according to claim 1, characterized in that the control meansare connected to another orifice in the pulp line between the pumpswhich orifice is provided with means for altering the through-flow.

4. Device according to claim 1, including a reference device for stockpressure in the headbox, the measuring cushions and possibly plus anegative pressure addition for vacuum running at low speeds, the outputsignals of said measuring means and reference means together forming anerror signal which is fed to the control means in the headbox measuringthe stock pressure in 5 means of the driving means for the mixture pump.

eluding fluid stock pressure plus pressure of any air

2. Device according to claim 1 characterized in that said orifice forobtaining a pressure drop comprises a Venturi tube.
 3. Device accordingto claim 1, characterized in that the control means are connected toanother orifice in the pulp line between the pumps which orifice isprovided with means for altering the through-flow.
 4. Device accordingto claim 1, including a reference device for stock pressure in theheadbox, the measuring means in the headbox measuring the stock pressureincluding fluid stock pressure plus pressure of any air cushions andpossibly plus a negative pressure addition for vacuum running at lowspeeds, the output signals of said measuring means and reference meanstogether forming an error signal which is fed to the control means ofthe driving means for the mixture pump.